The present invention relates to a contact shoe device for a circuit breaker, such as a molded case circuit breaker or an earth leakage breaker, and especially, to a contact shoe device with bridging movable contact shoes.
FIG. 5 shows a vertical sectional view of a three-pole circuit breaker in the ON state, which includes a contact shoe device of this kind. In this figure, a pair of fixed and opposing contact shoes 2 and 3 extending in a longitudinal direction is arranged in each electric path space in a mold case 1, wherein the electric path spaces are partitioned by partition walls for different polarities. Fixed contact shoes 2 and 3 have a pair of fixed contacts 2a and 3a, respectively, attached to bottom end surfaces thereof, and the fixed contact shoe 2 includes a built-in power supply terminal 4. A thermal and electromagnetic overcurrent tripping device 5 is arranged on the fixed contact shoe 3, though its internal configuration is not shown. The fixed contact shoe 3 is connected to a load terminal 6 via the overcurrent tripping device 5.
In the ON state shown in FIG. 5, a movable contact shoe 7 bridges the fixed contact shoes 2 and 3, and movable contacts 7a at opposite ends contact the fixed contacts 2a and 3a, respectively. The movable contact shoe 7 is guided and held by a movable contact shoe holder 8 formed of an insulating material, so as to slide vertically as shown in FIG. 5, while the movable contact shoe holder 8 is guided in the mold case 1 so as to slide vertically as shown in FIG. 5. The movable contact shoe 7 is urged against the fixed contact shoes 2 and 3 by a contact spring 9 formed of a compression coil spring and inserted between the movable contact shoe 7 and the bottom of the mold case 1 to apply predetermined contact pressure between the fixed and movable contacts. Arc-extinguishing chambers 10 formed of multiple arc-extinguishing grids laminated via gaps are arranged at front and rear sides of the movable contact shoe 7. The movable contact shoe holder 8, which holds the movable contact shoe 7, is opened and closed by a switching lever 11 located in the central polarity section. The switching lever 11 is supported on the mold case 1 so as to rotate around a switching shaft 12, and is opened and closed by a switching mechanism 13.
In such a circuit breaker, current flows from the power supply terminal 4 through the fixed contact shoe 2, movable contact shoe 7, fixed contact shoe 3, and overcurrent tripping device 5 to the load terminal 6, in this order. Then, when the operation handle 14 of the switching mechanism 13 is turned to become the OFF state, the switching lever 11 is rotated clockwise as shown in FIG. 5 to push the movable contact shoe holder 8 down against the contact spring 9. Thus, the movable contact shoe 7 is separated from the fixed contact shoes 2 and 3 to open the electric path.
In addition, when high current, such as short-circuit current, flows through the circuit breaker, the movable contact shoe 7 is driven downward in FIG. 5 by electromagnetic repulsion effected by contact between the fixed contacts 2a and 3a and the movable contact 7a, and by electromagnetic repulsion between the conductors of the fixed contact shoes 2 and 3 and the conductor of the movable contact shoe 7, which are arranged in parallel. The movable contact shoe 7 is thus quickly separated from the fixed contact shoes 2 and 3 as indicated by the dotted line. The overcurrent tripping device 5 is then actuated to operate the switching mechanism 13, which rotationally and rapidly drives the switching lever 11 clockwise due to energy stored in a switching spring (not shown). Consequently, the movable contact shoe 7 is held at a separated position via the movable contact shoe holder 8. At this point, arc 15 is generated between the fixed contacts 2a and 3a and the movable contact 7a, and expands as the movable contact shoe 7 is separated. The arc is finally drawn into the arc-extinguishing chamber 10 and extinguishes.
In the conventional circuit breaker shown in FIG. 5, the contact spring 9 is compressed when the movable contact shoe 7 is separated, so that as the separation of the movable contact shoe 7 proceeds, reaction force from the contact spring 9 increases to gradually reduce the separation speed of the movable contact shoe 7. To minimize the decrease in the separation speed, the number of windings in the contact spring 9 must be increased to reduce its spring constant. In this case, however, the contact spring 9 becomes longer correspondingly. However, when the contact spring is elongated, the interval between the movable contact shoe 7 and the bottom of the mold case 1 must be increased. As a result, the mold case 1 becomes higher, which hinders the size reduction of the circuit breaker.
The present invention solves these problems, and an object of the invention is to provide a circuit breaker which can reduce spring constant of a contact spring while preventing an increase in size of a mold case associated with contact spring installation.
Another object of the invention is to provide a circuit breaker as stated above, which can increase separation speed of the movable contact shoe without hindering the size reduction of the circuit breaker.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the objective, the present invention according to the first aspect provides a contact shoe device for a circuit breaker, which comprises pairs of fixed contact shoes, with each pair opposing to each other and having a different polarity, and movable contact shoes, each bridging the fixed contact shoes. The movable contact shoe is pressed against the fixed contact shoes by contact springs inserted between the movable contact shoe and a mold case to close the electric path for each polarity, while the movable contact shoe is separated from the fixed contact shoes against the contact spring when the electric path is open. The contact spring is formed of a compression coil spring, and two contact springs are provided for each of the movable contact shoes.
When the number of windings in the contact spring formed of the compression coil spring increases to reduce the spring constant and reaction force applied by the contact spring when the movable contact shoe is separated, the contact spring becomes longer than that in conventional models. This requires that the spring contracting distance is increased to apply the same contact pressure as in the prior art while the circuit is closed. Thus, in the conventional configuration where the contact spring is inserted between the bottom surface of the movable contact shoe and the mold case, when an attempt is made to insert a long contact spring in a condition that an insertion space is unchanged, adjacent windings come to contact each other, and inhibit separation of the movable contact shoe from the fixed contact shoes over the required distance.
Thus, in the first aspect, each movable contact shoe has two contact springs, each of which is formed of a compression coil spring to evenly distribute the load on the movable contact, i.e. half the load when using a single contact spring. This reduces the wire diameter of each contact spring and increases the distance of spring contraction before the windings come into contact with each other, thereby enabling separation of the movable contact shoe over the required distance without significantly increasing the insertion space for contact springs, and regardless of the increase in the number of windings to reduce the spring constant.
In addition, according to the present invention in the second aspect, the contact springs are arranged on opposite sides of the movable contact shoe, and a spring holder is interposed between the movable contact shoe and contact springs. The upper ends of the contact springs extend above the movable contact shoe. Consequently, as compared to the insertion of the contact spring between the bottom surface of the movable contact shoe and the mold case, the insertion space for the contact spring can be more easily enlarged even at the same mold case height.
Conversely, in a third aspect, each of the contact springs may be formed of a torsion spring. Since the torsion spring has a constant height despite the increase in the number of windings, the spring constant of the contact spring can be reduced without increasing the height of the mold case.